1. Field of the Invention
The present invention relates to a camera system which can be connected to a flash unit or a data back.
2. Related Background Art
There are already known various cameras to which various accessories such as a flash unit or a data back can be connected. With recent advances in the camera systems, various functions are incorporated not only in the camera itself but also in the accessories to be connected thereto, for increasing the functions not only of the accessory but also of the camera itself. Examples of such functions include auxiliary light control or flash synchronization with the trailing shutter curtain in the TTL light control with a flash unit, and an auto bracketing function for stepwise correction of exposure or an extended exposure function in a data back. Such functions are not simply achievable only in the accessory, but require information transfer of the function to the camera body, and reception of various data therefrom. For achieving such functions, setting means for such functions are provided on the accessory, and the state of the function setting is electrically transmitted to the camera through certain electrical contacts, and data are received from the camera. Since the number of such contacts is limited in space and cost, there is generally employed a method of exchanging necessary data collectively in digital form, in serial data communication.
In order to effect such serial data communication between the accessory and the camera body, the power supply has to be turned on in both units. In cameras, simple power supply which activates the camera only during the turned-on state of a power switch has become unpopular, because of trouble when the user forgets to turn off the switch. For this reason, there is commonly employed a structure containing a so-called power supply holding timer which automatically turns off the power supply to a negligible level after a predetermined time and restores the power supply in response, for example, to a half-stroke depression of the shutter release button. Stated differently, the power supply is turned off if the camera is left untouched. Consequently, when the buttons of the accessory are manipulated for executing the functions incorporated therein, serial data communication for setting or executing such functions is not conducted between the camera and the accessory in a state in which the power supply holding timer is turned off. In order to effect such communication it is necessary to manipulate the power supply activating means of the camera body each time. Japanese Laid-open Patent Sho 61-260230 for an electronic flash and Japanese Laid-open Patent Sho 62-115141 for an activating device for a camera system have proposed solutions to this drawback, by providing means for activating the power supply of the camera from the accessory, and means for activating the camera body from the accessory, when data communication with the camera body is required. Such structures can prevent the above-mentioned drawback as data communication is enabled by automatic activation of the power supply of the camera for example at the function setting in the accessory.
However, there still exists another drawback in the convenience of use. More specifically, in the structure in which the camera body can be activated by the manipulation of the accessory when it has no data communication with the camera body, the power supply holding timer of the camera is turned off after a predetermined time while the user continues the setting operation on the accessory, whereby the setting operation becomes wasted. For solving this drawback, it is possible to send the signal of the activating means of the accessory also to the power supply holding timer during data communication, thereby constantly retriggering the power supply holding timer. However, since such activating means is often composed in a common signal line with switch means to be turned on by a half-stroke depression of a shutter release button which constitutes power supply activating means in the camera and which also triggers other operations such as automatic focusing, there may result other drawbacks such as the activation of the automatic focusing by the manipulation of a button in the accessory.